In the last few years, mobile information and telecommunication devices such as Notebooks and personal digital assistants have experienced fast development. The corresponding devices are becoming lighter and more efficient. Recently, flat panel displays are becoming more and more popular for such devices. Currently, liquid crystal displays (LCDs) are used as flat panel displays, although LCDs have some disadvantages, e.g. the need for background lighting and a limited viewing angle.
Besides liquid crystals, Organic Light Emitting Diodes, so-called “organic LEDs”, or “OLEDs”, can be used in flat panel displays. Such organic LEDs have a higher luminous efficiency and an increased viewing angle. The basic feature of the organic LED is the electroluminescence of specific organic materials. The specific organic material determines in a first approximation the color, i.e. the wavelengths, of the light emitted by the corresponding organic LED.
FIG. 1 shows a schematic view of a conventional organic LED 100. The common organic LED 100 comprises a substrate 101, which is usually made of glass or a similar transparent material. An anode layer 102 is formed on the substrate 101. Preferably the anode layer 102 is made of a material having a relatively high work function and is substantially transparent for visible light. Therefore, a typical material for the anode layer 102 is indium tin oxide (ITO). A layer 103 of electroluminescent material is formed on the anode layer 102, serving as the emitting layer 103 of the organic LED 100. Common materials for forming the emitting layer 103 are polymers like Poly(p-phenylenvinylen) (PPV) and molecules like tris (8-oxychinolinato) aluminum (Alq3). In the case of molecules the emitting layer 103 typically comprises several layers of the molecules. A cathode layer 104 of material having a lower work function like aluminum (Al), calcium (Ca) or magnesium (Mg) is formed on the emitting layer 103. The cathode layer 104 and the anode layer 102 are in operation connected to a power supply 105.
The basic principles of electroluminescence and, thus, of the organic LED are the following. The anode layer 102 and the cathode layer 104 inject charge carriers, i.e. electrons and holes, into the emitting layer 103, i.e. the active layer. In the emitting layer 103 the charge carriers are transported and the charge carriers of opposite charge form so called excitons, i.e. excited states. The excitons decay radiatively into the ground state by generating light. The generated light is then emitted by the organic LED through the anode layer 102, which is made of transparent material like ITO. The color of the generated light depends on the material used for the organic layer 103.
Furthermore, a so-called multilayer organic LED is known. The multilayer organic LED comprises a plurality of cathode layers and/or a plurality of organic layers and/or a plurality of anode layers. By using a plurality of organic layers, the efficiency of the organic LED can be increased compared to the organic LED comprising a single organic layer. The boundary surface between two organic layers of the plurality of organic layers can act as a barrier which reduces the current flow through the diode for at least one charge carrier type (electrons or holes). Therefore, the at least one charge carrier type accumulates at the boundary surface and thus the recombination probability of the electrons and the holes is increased leading to a higher efficiency of the organic LED.
In “Application of an Ultrathin LiF/Al bilayer in Organic Surface-Emitting Diodes”, L. S. Hung et. al., Applied Physics Letters Vol. 78, No. 4 (22, Jan. 2001), pp. 544-546, an organic LED is disclosed in which the generated light is emitted through the cathode of the organic LED instead of emitting the light through the anode. A schematic illustration of such an organic LED is shown in FIG. 2. An indium tin oxide (ITO) layer 201 is provided as a substrate of the organic LED 200. An α-naphtylphenylbiphenyl diamine (NPB) layer 202 is formed on the ITO substrate 201 to act as a hole transport layer. Underneath the ITO substrate 201 a reflecting silver mirror 209 is formed. An Alq3 layer 203 is formed on the hole transport layer 202 to act as an electron-transport/emissive layer. Further, a cathode 204 is formed on the Alq3 layer 203.
The cathode 204 is formed by a plurality of cathode layers as a so-called multilayer cathode structure which is optically transmissive and which is effective as an electron-injecting contact for the organic LED. The multilayer cathode structure comprises an ultrathin layer of lithium fluoride (LiF) 205, an Al layer 206 as the electron-injecting contact and a silver layer 207 for sheet resistance reduction. Furthermore, a transparent dielectric layer 208 for enhancement of optical transmission is formed on the multilayer cathode structure. This transparent, dielectric layer 208 is used for increasing the efficiency of emitting the light through the cathode 204, i.e. the multilayer cathode structure.
In this organic LED, generated light is emitted through the cathode 204. Such an organic LED is also referred to as a top-emitting organic LED. The top-emission is possible, since the cathode 204 comprises a LiF/Al bilayer. A reasonable thickness of the LiF layer 205 is given to be about 0.3 nm and a reasonable thickness of the Al layer 206 to be between 0.1 nm and 1.0 nm. For the dielectric layer 208, also referred to as a “capping layer”, Alq3 or MgO can be used.
One disadvantage of this top-emitting organic LED is that the thickness of the refractive dielectric layer 208 has to be adjusted to the color of the organic LED, i.e. the wavelength of the light emitted from the specific organic LED. That is, each organic LED emitting a different color has to have a different thickness of the refractive layer 208 in order to increase the efficiency of the organic LED. Thus, when a flat panel display comprising organic LEDs emitting different colors is manufactured, the thickness of the refractive dielectric layers 208 for organic LEDs of each color is different. To schematically illustrate this disadvantage, FIG. 2 is divided into three parts 2a, 2b, 2c. FIG. 2a depicts an organic LED for emitting light of a relatively short wavelength, e.g. blue light, and has a refractive dielectric layer 208 that is relatively thin. FIG. 2b depicts an organic LED for emitting light of a medium wavelength, e.g. green light or yellow light, and has a refractive dielectric layer 208 that is thicker than the refractive dielectric layer 208 shown in FIG. 2a. FIG. 2c, depicts an organic LED for emitting light of a relatively long wavelength, e.g. red light, and has a refractive dielectric layer 208 that is thicker than those shown in FIGS. 2a and 2b. With increasing wavelength a longer optical pathlength in the refractive dielectric layer 208 is required to achieve the object of the refractive dielectric layer 208, that is to increase the light output of the organic LED. Furthermore, the thicknesses of the α-naphtylphenylbiphenyl diamine (NPB) layer 202 and of the Alq3 layer 203 of the organic LED have to be adapted as well.
Therefore, in a display with a plurality of red, green and blue pixels, wherein each pixel is realized by a single organic LED, refractive dielectric layers 208 having a different layer thickness for each color have to be deposited on the organic LEDs. This leads to a complex and hard to handle production method of the display comprising a plurality of organic LEDs, i.e. for every thickness of the refractive dielectric layer 208 a separate process step is necessary.
Another disadvantage of such an organic LED is that, although the refractive dielectric layer 208 enhances the light output, i.e. the luminous intensity, the contrast ratio of the organic LED is still relatively low.
It is desired to overcome at least some of the disadvantages of conventional organic LEDs.